1. Field of the Invention
The present invention relates to a semiconductor substrate, and a process for producing the semiconductor substrate. More particularly, the present invention relates to a semiconductor substrate suitable for dielectric isolation, and electronic devices and integrated circuits formed in a monocrystalline semiconductor layer on an insulator, and to a process for producing such a semiconductor substrate.
2. Related Background Art
Formation of monocrystalline Si semiconductor layer on an insulator is well known as silicon-on-insulator technique (SOI). Many investigations have been made thereon since the devices made by the SOI technique have many advantages which are not achievable with a bulk Si substrate for preparing usual Si integrated circuits. The advantages brought about by the SOI technique are as below:
1. Ease of dielectric separation, and practicability of high integration, PA1 2. High resistance against radioactive rays, PA1 3. Low floating capacity, and practicability of high speed operation, PA1 4. Practicability of omission of a welling step, PA1 5. Practicability of prevention of latching-up, PA1 6. Practicability of thin film formation for complete depletion type field effect transistor, PA1 (1) methods comprising steps of oxidizing a surface of an monocrystalline Si substrate, forming an aperture in the oxidized layer to uncover partially the Si substrate, growing Si epitaxially in a lateral direction using the uncovered Si as the seed to form an monocrystalline Si layer on the SiO.sub.2 (deposition of Si on SiO2), and PA1 (2) methods of forming SiO.sub.2 under a monocrystalline Si substrate by use of the monocrystalline Si substrate itself as the active layer (no deposition of Si layer). PA1 1. A surface of a monocrystalline Si substrate is etched anisotropically to form V-shaped grooves on the surface. An oxide film is formed thereon. On the oxide film, a polycrystalline Si layer is deposited in a thickness that is nearly the same as the Si substrate. Then the Si substrate is abraded at the backside to form dielectrically separated monocrystalline Si regions surrounded by the V-shaped grooves on the thick polycrystalline Si layer. Although this technique gives satisfactory crystallinity, it involves problems in control and productivity in the process of depositing polycrystalline Si in a thickness of as much as several hundred .mu.m and in the process of abrading the monocrystalline Si substrate from the backside to leave only the separated active Si layer. PA1 2. An SiO.sub.2 layer is formed on a monocrystalline Si layer by oxygen ion implantation. This method is called SIMOX (separation by ion implanted oxygen). This process exhibits excellent coherency with the Si process, and is the most highly developed technique at the moment. However, the process requires implantation of oxygen ions as much as 10.sup.18 ions/cm.sup.2 to form the SiO.sub.2 layer, and the implantation takes long time, so that the productivity is not high, and the wafer cost is high. Furthermore, the product has many remaining crystal defects, and does not have satisfactory quality for industrial production of minority carrier devices. PA1 3. The SOI structure is formed by dielectric separation by oxidation of porous Si. In this method, on a surface of a P-type monocrystalline Si substrate, an N-type Si layer is formed in an island shape by proton ion implantation (Imai, et al.: J. Crystal Growth, vol. 63, 547, (1983)), or by epitaxial growth and patterning, then the P-type Si substrate only is made porous by anodization in an HF solution so as to surround the island-shaped Si regions, and the N-type Si islands are separated dielectrically by accelerated oxidation. This method has disadvantage that the freedom in device design is frequently restricted since the Si region to be separated has to be decided prior to the device production process.
and so forth.
In order to realize the aforementioned advantages in device characteristics, investigations have been made on the methods of forming the SOI structure for the last few decades. The results of the investigations are collected in the literature, for example: Special Issue: "Single-crystal silicon on non-single-crystal insulators"; edited by G. W. Cullen, Journal of Crystal Growth, Vol. 63, No. 3, pp. 429-590 (1983).
Formerly, the SOS technique (silicon-on-sapphire) is known which forms heteroepitaxial Si on monocrystalline sapphire substrate by CVD (chemical vapor deposition). This technique, although successful as completed SOI technique, is not widely applied because of many crystal defects caused by insufficient matching of the lattice at the interface between the Si layer and the underlying sapphire substrate, migration of aluminum from the sapphire substrate to the Si layer, and, above all, the high cost of the substrate and difficulty in enlarging the area thereof.
In recent years, the SOI structure without use of the sapphire substrate is going to be realized. This attempt is made in two methods:
The means for practicing the methods (1) above include direct epitaxial growth of monocrystalline Si layer in a lateral direction by CVD; deposition of amorphous Si and subsequent epitaxial growth in solid in a lateral direction by heat treatment; growth of amorphous or polycrystalline Si into monocrystalline layer by melting-recrystallization by focusing thereon an energy beam such as electron beam and laser light; and zone melting recrystallization by a long heater. These methods have both advantages and disadvantages, still involving problems in controllability, productivity, uniformity, and quality of the products. Therefore, none of these methods has been practiced industrially.
For example, the CVD process requires sacrificial oxidation. The solid growth results in low crystallinity. The beam annealing process involves problems in treating time, control of superposition of the beam and focus adjustment. Among the above methods, zone melting recrystallization is most highly developed, and has been employed for experimental production of relatively large integrated circuits. This method, however, still causes crystal defects such as subgrain boundary, etc., and does not give a minority carrier device.
The methods (2) above, which does not use the Si substrate as the seeds for epitaxial growth, are practiced in the three ways below:
For example, Japanese Patent Publication No. 53-45675 discloses a process for forming SOI in which a monocrystalline silicon layer is grown on a porous layer or a porous insulating layer. In this process, a porous layer is heat-treated in an oxidizing atmosphere to give the layer of high resistance, thereby obtaining an SOI structure. This process, although proposed more than 10 years ago, has not been practically employed. This is because the porous material is thermally instable and nonuniform in structure. In the porous material, extremely fine pores are formed in monocrystalline silicon. The thickness of monocrystalline silicon layer remaining between the pores can be made to be in the range of as small as from several nm to several tens of nm. Such porous silicon is oxidizable in a much larger reaction velocity than normal monocrystalline silicon. By utilizing this phenomenon, the SOI can be prepared by selectively oxidizing the porous lower portion of the monocrystal layer. This method, however, involves the disadvantage that satisfactory insulating layer cannot readily be prepared owing to residual silicon in the insulating layer and variation of the thickness of the oxide layer which are caused by nonuniformity in size of the silicon regions remaining in the porous layer. This method involves a further disadvantage that the porous material comes to have coarser structure during the heat treatment at the high temperature required for oxidation, whereby the rate of oxidation falls. Furthermore, if the porous silicon or a high-resistance porous material remains on the substrate, it induces denaturation such as structural change or coarsening of the porous structure in heat treatment for formation of an element on the substrate, which tends to cause warpage or distortion of the substrate.
On the other hand, a light-transmissive substrate typified by glass allows Si to grow only into an amorphous or polycrystalline layer under the influence of the disorderness of the crystal structure thereof, and is unsuitable for production of devices of high performance. Simple deposition of Si on such a substrate will not give excellent monocrystal layer because of the amorphous structure of the substrate. The light-transmissive substrate is important in constructing a contact sensor as a light-receiving element, a projection type liquid crystal image displaying apparatus, and the like. In order to provide a sensor or a display apparatus with image elements (picture elements) in higher density, higher resolution, and higher fineness, extremely high performance of the driving element is required. Therefore, the element on a light-transmissive substrate have to be made from monocrystal layer having excellent crystallinity.
In other words, amorphous Si or polycrystalline Si will not generally give a driving element which exhibits the satisfactory performance required or to be required in the future because of many defects in the crystal structure.
On a light-transmissive substrate, however, none of the above methods for a monocrystalline Si substrate is suitable for forming an excellent monocrystal layer.